Critical frequency stability requirements placed on modern communications equipment have forced the development of phase locked loop technology and associated hardware and circuits. For example, a single-loop synthesized frequency generator provides thousands of accurate channels through frequency synthesis using a single crystal oscillator. The generator is tuned from channel to channel using a phase locked loop containing a voltage controlled oscillator VCO) to generate the desired output signal.
One of the more common types of VCO is tuned using an abrupt junction voltage variable capacity diode (VARACTOR). By changing the input tuning voltage to the resonant circuit containing the varactor, the output frequency of the generator may be varied. However, in conventional phase locked loop circuits required to operate over an appreciable frequency range, the loop gain varies excessively with frequency changes. In many applications, such variation in loop gain is unacceptable. Further, the relationship between output frequency and input tuning voltage is non linear so that retuning the VCO causes the loop gain within the phase locked loop to vary.
Various prior art methods have been utilized to overcome this problem. One such method is found in U.S. Pat. No. 3,538,450, Andrea et al., which discloses a phase locked loop circuit in which a reference signal source is controlled digitally. In Andrea et al., one or more of a bank of fixed capacitors having values which increase in a binary order are selected for controlling the frequency of a signal source. A binary counter, responsive to the output of a clock pulse source counts up or down to control a switch, which in turn adds to or subtracts from the oscillator circuit discrete increments of capacitance from the bank of capacitors. Thus the amount of capacitance coupled into the oscillator circuit is determined from the number of pulses clocked into the counter. However, the addition or subtraction of discrete increments of capacitance is a linear function which can only approximate the non-linear gain variations created by frequency changes of the generator.
Accordingly, there is a need for a phase locked loop circuit which automatically compensates for non-linear gain variations within the loop which are caused by changing the output frequency of the generator.